I thought that the S-Class Coupe was going to replace the CL. But you're right, it is still a concept, so nothing has been explicitly confirmed.

EDIT: This is indeed going to be the S-Class Coupe which will replace the CL

How do we know that for sure?

Idk, Mercedes said themselves that this was still only a concept meant to show the world what Mercedes can do. It's not replacing anything. I quoted them on the first page of this thread - Direct from their official FB page.